HELIUM PRESSURE BROADENING OF AMMONIA INVERSION TRANSITIONS FROM 10 TO 35 K

Helium pressure broadening cross sections for the (J,K)=(1,1), (2,2) a nd (3,3) inversion transitions of ammonia were measured at temperature s from 10 to 35 K. Measurements were taken in a quasiequilibrium cell using the collisional cooling technique. In contrast to the situation at room temperature where there is little state-to-state variation in helium pressure broadening cross sections, at low temperature the (3,3 ) cross sections were found to be, on average, three times larger than the (1,1) cross sections while the (2,2) cross sections were twice as large as the (1,1) values. The major factor determining the relative size of the low temperature cross sections appears to be the contribut ion from inelastic collisions into lower energy levels. The relative s ize of the cross sections thus scales with the number of rotational le vels underlying a given (J,K) state. Cross sections were also calculat ed for comparison with the experimental data using three existing NH3- He potential surfaces. Overall, none of the three surfaces gave consis tent agreement with the experimental results for all three observed tr ansitions. The discrepancy between experiment and theory was greatest for the (3,3) data. This may be a manifestation of a previously observ ed tendency of NH3-He potential surfaces to underestimate the contribu tion from parity-conserving collisions in calculated state-to-state ra tes. (C) 1997 American Institute of Physics. [S0021-9606(97)03944-5].